The incredible is that if I go back to 777 folder, this fails again :
serveur private # rm /etc/vz/conf/777.conf
serveur private # mv 703 777
serveur private # vzctl set 777 --applyconfig vps.basic --save
Saved parameters for VPS 777
serveur private # vzctl start 777
Warning: distribution not specified default used /etc/vz/dists/default
Starting VPS ...
vzquota : (error) Quota on syscall for 777: No such file or directory
vzquota on failed [3]
serveur private #
Very strange isn't it ?